Preparation of indigo-dyed cotton denim fabrics and garments

ABSTRACT

Disclosed herein is the preparation of indigo-dyed cotton denim fabrics suitable for use in making cotton denim garments and other denim articles. These indigo-dyed fabrics are prepared from cotton warp yarn which has been pre-treated with an emulsion copolymer prior to being contacted with an aqueous dye liquor comprising a dispersion of an indigo dyestuff. Such copolymer-treated cotton warp yarn can be woven or knitted into cotton denim griege fabrics along with untreated cotton weft yarn. Such griege denim fabric can then be indigo-dyed using the aqueous dye liquor. Alternatively, the emulsion copolymer-treated cotton warp yarn can be indigo-dyed by contact with the aqueous dye liquor before this warp yarn is incorporated into denim fabric along with the untreated cotton weft yarn. The cotton denim fabric produced by either method has the appearance of conventional ring-dyed indigo fabrics.

FIELD

The present development relates to the preparation of indigo-dyed cottondenim fabrics and garments. Such denim fabrics and garments have anon-uniformly colored appearance.

BACKGROUND

Denim garments are produced from denim fabric. Denim fabric is generallya 3×1 twill woven structure where the warp yarns are indigo dyed and theweft (or filling) yarns are undyed cotton. The English cotton count (Ne)of yarns used to make denim is usually 7/1's.

The unique “salt and pepper” effect of denim is created by the dyed warpyarns mingling with undyed weft yarns. The weft is packed on the loom tobe hidden on the technical back of the fabric, thus making the inside ofjeans look whiter. Due to the specific nature of the end product, theindigo dyed yarns in denim must be dyed in yarn form, not as a fabric.This yarn dyeing takes (for majority of denim) two forms. Rope dyeing isthe most common process. Slasher dyeing is less frequently used and isaimed mostly at higher end or smaller production lots of denim.

The dyeing procedure is designed to best apply a ring dyed effect oncotton yarns with indigo, vat and sulfur dyes. These dye classes requirea reduction/oxidation potential (−mV=600 to 800) and high loading ofcaustic (pH approx. 12 to 13) to produce a water dispersable andcotton-substantive dyestuff. The main purpose of ring dyeing is tocreate a layer of dyestuff on the outside perimeter of the yarn crosssection that can be removed when stonewashing or other garment washingsteps are performed post dyeing. The theory is that a small quantity ofdyed surface yarn is removed by pumice/enzymes/etc. which then revealsthe undyed or whiter cotton beneath. This removal of color creates the“character” and wash down look which retailers and their consumersexpect of denim products.

Rope dyeing is described in Schoots; U.S. Pat. No. 7,201,780; IssuedApr. 10, 2007. Yarn is gathered in “ropes” made of 300 to 400 yarns.These ropes (25 to 50 ropes/machine) are sent through a continuous ropedyeing machine made up of one circulating dye bath separated by 5 to 8boxes. The ropes travel through the dye baths for approximately 15 to 20seconds submerged allowing the leuco form of indigo dye to paint anouter layer of color onto the yarn. This yarn then proceeds into a“skying” segment where the leuco indigo (bright yellow) is transformedto the oxidized blue indigo with air. The process repeats up to 8 timesto continue to build color yield on the perimeter of the yarn. Depth ofthis penetration is referred to as “pop”, which can be also controlledwith manipulation of the textile auxilary chemicals in the bath, caustic(NaOH) or reducing agent such as sodium hydrosulfite, sodiumborohydride/bisulfite combination.

A slasher continuous dyeing machine uses the exact same dyeing mechanismand process as rope dying, but with less yarn. Only the amount of yarnused in a creel for a loom runs through the machine (approx. 1,600 yarnswide<2 meters). This allows for more accurate creation of specificcolors. This process also allows for direct placement of final yarndirectly into a loom.

The delicate balance of caustic, reducing agent, indigo (and vat orsulfur topping/bottoming), time in dye bath, skying time, etc. is easilydisrupted. Off quality rope dyed yarn is relatively high quantity versusother dyeing forms. Some manufacturers warehouse the different shades ofresulting product with a “555 sorting” method. There are risks to thismethod leading to the inevitable lack of supply of one of the 555 boxes.

The variation in final color of indigo-dyed yarns and denim fabrics isremarkably broad considering that only a single type of dyestuff isused. Critical elements in final denim is “red shade” of the blue color,depth of shade, contrast, penetration, washfastness of resulting denim,crockfastness, character and hand.

Given the foregoing considerations, it would be advantageous to developalternative methods for production of indigo-dyed denim fabrics andgarments with such alternative methods providing improved consistency,reproducibility and predictability compared to the delicate indigodyeing processes existing today. Such alternative denim preparationmethods would also desirably minimize variations in final denim colordue to unintended variations in depth of shade. A further advantageprovided by such methods could also be the elimination of the ropedyeing process altogether, with the desired denim fabric (and garments)being dyed in batch jet or similar machines instead.

SUMMARY

In one aspect, the present development relates to a method for preparingindigo-dyed denim fabric. In the first step of such a method,emulsion-treated warp yarn is prepared by contacting cotton yarn with acellulose-reactive emulsion copolymer and by thereafter curing theemulsion copolymer.

In the second step of such a method, the emulsion-treated cotton warpyarn is woven or knitted into denim fabric along with untreated cottonyarn as the weft yarn. In this manner, griege denim fabric is preparedhaving emulsion-treated cotton warp yarn in the warp direction only.

In the third step of such a method, the griege denim fabric so preparedis contacted with an aqueous dispersion of an indigo dyestuff material.This contact with the dyestuff-containing dispersion is carried outunder conditions which are sufficient to preferentially color the outercross-sectional portions of the emulsion copolymer-treated warp yarn toa greater extent than the indigo dyestuff material colors the outercross-sectional portions of the untreated cotton weft yarn.

This method thus provides indigo-dyed denim fabric having anon-uniformly colored appearance. In a preferred embodiment of such amethod, the non-uniformly colored indigo-dyed denim fabric can besubjected to further dyeing by contacting the denim fabric with anadditional dyestuff material of the indigo, vat, cationic, azoic,napthol, reactive, direct, sulfur, mordant, disperse or acid type.

In another aspect, the present development relates to an alternativemethod for preparing indigo-dyed denim fabric. In the first step of suchan alternative method, emulsion copolymer-treated warp yarn is preparedby contacting cotton yarn with a cellulose-reactive emulsion copolymerand by thereafter curing the emulsion copolymer.

In a second step of such an alternative method, the emulsioncopolymer-treated warp yarn is contacted with an aqueous dispersion ofan indigo dyestuff material before the warp yarn is fashioned intofabric. Contacting of the copolymer-treated warp yarn with the dyedispersion is carried out under conditions sufficient to color the outercross-sectional portions of the emulsion copolymer-treated warp yarn.

In a third step of such an alternative method, the emulsioncopolymer-treated, colored warp yarn is woven or knitted into denimfabric along with untreated cotton yarn as the weft yarn. The denimfabric so prepared thus has emulsion copolymer-treated, colored cottonyarn in the warp direction only. This provides indigo-dyed denim fabrichaving a non-uniformly colored appearance.

In yet another aspect, the present development is directed toindigo-dyed cotton denim fabric comprising a plurality of cotton warpyarns and a plurality of cotton weft yarns. The cotton warp yarn of thefabric is treated with a cellulose reactive emulsion copolymer and thencontacted with an aqueous dispersion of an indigo dyestuff materialunder conditions sufficient to color the outer cross-sectional portionsof this emulsion copolymer-treated cotton warp yarn. The cotton weftyarn of the fabric is untreated prior to optional contact of this cottonweft yarn with an aqueous dispersion of indigo dyestuff material.

DETAILED DESCRIPTION

The present development is directed to the preparation of indigo-dyeddenim fabric comprising woven or knitted cotton yarn. Treatment of thecotton yarn used, such as by pretreatment and/or indigo dyeing,preparation of the denim fabric from the yarn and post-treatment of thefabric by dyeing or other operations are all described in detail asfollows:

Cotton Yarn

Cotton yarn refers to a double or multi-stranded filament made bytwisting or otherwise bonding cotton staple fibers together to make acohesive thread. Twisting fibers into yarn is, of course, the processcalled spinning.

The cotton staple fibers can be spun into yarn in the form of single plyor multi-plied yarns. Cotton staple fibers which form such yarnstypically range from about 1.0 to about 3.0 denier per filament (dpf)and have a staple length range of from about 0.5 to 8.0 cm.

It is well known that cotton fibers can be combined with other fibertypes when fashioned and used in the form of yarns. However, the bestpre-treating, dyeing and fabric formation results obtained using themethods described herein are achieved when the yarns used contain nofibers other than cotton. Accordingly, yarns which are 100% cotton arepreferred for use in the present method.

Cotton yarn which has been, or is to eventually be, incorporated intodenim fabric in the warp direction is referred to herein as “warp yarn”or “cotton warp yarn”. Conversely, cotton yarn which has been, or is toeventually be, incorporated into denim fabric in the weft (or fill)direction is referred to herein as “weft yarn” or “cotton weft yarn”.

Cellulose-Reactive Emulsion Copolymer

The cotton warp yarn used in the methods and fabrics herein are, priorto dyeing and incorporation into denim fabrics, contacted and treatedwith a selected type of cellulose-reactive emulsion copolymer. Suchemulsion copolymers include those which have conventionally been used astextile finishing agents. Such emulsion copolymers include thosedescribed in detail in U.S. Patent Publication No. 2011/0005008, whichis incorporated by reference herein in its entirety.

Suitable types of cellulose-reactive emulsion copolymers for use intreating the cotton warp yarn include vinyl ester-based, acrylic-based,styrene/acrylic-based and/or styrene/butadiene-based emulsioncopolymers. Such copolymers typically can also contain minor amounts ofcross-linking or emulsion stabilizing co-monomers. Such co-monomers can,for example, in and of themselves or in combination with externalcross-linking agents, make the emulsion copolymers used hereincellulose-reactive.

One preferred type of emulsion copolymer comprises the vinyl ester-basedcopolymers selected from vinyl acetate-ethylene copolymers, vinylacetate-vinyl versatate; vinyl acetate-acrylic copolymers, andcombinations of these copolymer types. Vinyl acetate-ethylene (VAE)emulsion copolymers are well-known. Such VAE copolymers useful hereincan comprise from about 60 wt % to about 95 wt % of vinyl acetate andfrom about 5 wt % to about 40 wt % of ethylene, based on total monomerstherein. More preferably, VAE copolymers will comprise from about 70 wt% to about 92 wt % of vinyl acetate and from about 8 wt % to about 30 wt% of ethylene, based on total monomers therein.

Another preferred type of emulsion copolymer for use in the methodherein comprises acrylic emulsion copolymers made of acrylic esterco-monomers. The alkyl acrylates that can be used to prepare the acrylicester copolymer emulsions include alkyl acrylates and alkylmethacrylates containing 1 to 12, preferably 1 to 10 carbon atoms in thealkyl group. The polymer backbone in the acrylic ester copolymer can beeither hydrophilic or hydrophobic and it can comprise polymerized softmonomers and/or hard monomers. The soft and hard monomers are monomerswhich, when polymerized, produce soft or hard polymers, or polymers inbetween. Preferred soft acrylic ester monomers are selected from alkylacrylates containing 2 to 8 carbon atoms in the alkyl group and includeethyl acrylate, propyl acrylate, n-butyl acrylate, and 2-ethylhexylacrylate. The hard acrylic ester monomers are selected from alkylmethacrylates containing up to 3 carbon atoms in the alkyl group andfrom non-acrylic monomers such as styrene and substituted styrenes,acrylonitrile, vinylchloride, and generally any compatible monomer thehomopolymer of which has a T_(g) above 50° C. Preferred acrylic estermonomers are selected from alkyl acrylates and methacrylates containing1 to 12 carbon atoms in the alkyl group, especially ethyl acrylate andbutyl acrylate.

The cellulose-reactive emulsion copolymer will frequently contain, inaddition to the main co-monomers, minor amounts of co-monomers which canprovide cross-linking with both cellulose hydroxyl moieties within thecotton fibers and cross-linking within the copolymer itself. Suchcross-linking co-monomers are unsaturated so as to polymerize into thecopolymer backbone and will also contain at least one functional groupcontaining nitrogen, oxygen or silicon atoms.

Thus the cellulose-reactive emulsion copolymers herein can comprise fromabout 0.1 wt % to about 10 wt %, based on total monomers in thecopolymer, of one or more ethylenically unsaturated cross-linkingco-monomers having, for example, at least one amide, epoxy, oralkoxysilane group. Examples of such suitable self cross-linkingco-monomers include N-methylol (meth)acrylamide and esters therof,N-vinylpyrrolidinone, dimethylaminoethyl acrylate, glycidyl acrylate,glycidyl methacrylate, allyl glycidyl ether, vinyl glycidyl ether,acryloxy-propyltri(alkoxy)silanes, methacryloxypropyltri(alkoxy)silanes,vinyltrialkoxysilanes, vinylmethyldialkoxysilanes and combinations ofthese cross-linkable co-monomers.

The cellulose-reactive emulsion copolymer can also contain, in additionto the main co-monomers and self cross-linking co-monomers, minoramounts of multifunctional external cross-linking co-monomers. Thus thecopolymers used herein can optionally comprise from about 0.1 wt % toabout 10 wt %, based on total monomers in the copolymer, of one of moreof these multifunctional cross-linking co-monomers. Examples of suitablemultifunctional cross-linking co-monomers include diallyl adipate,triallyl cyanurate, butanediol diacrylate, allyl methacrylate andcombinations thereof.

Cellulose-reactive emulsion copolymers used prior to dyeing to modifycotton warp yarn in accordance with the methods herein can frequently beselected from commercially available copolymer emulsions. Alternatively,suitable cellulose-reactive emulsion copolymers can be prepared inconventional fashion using known emulsion polymerization techniques andraw materials. In general, such emulsion copolymers can be prepared bypolymerizing appropriate co-monomers in appropriate amounts in anaqueous reaction mixture using conventional polymerization initiatorsand catalysts and conventional polymerization conditions. The copolymeremulsions so prepared can be stabilized with suitable emulsifiers(surfactants) and/or protective colloids.

Warp Yarn Pre-Treatment Conditions

The cellulose-reactive emulsion copolymers as described above are usedto contact, pre-treat and chemically modify the cotton warp yarnmaterial prior to an indigo dyeing operation. Such a procedure firstinvolves contacting the cotton warp yarn and emulsion copolymer toprepare a warp yarn/copolymer combination. This warp yarn/copolymercombination is then subjected to curing which serves to chemicallyanchor the copolymer to the cotton fibers in the warp yarn via reactionwith the hydroxyl groups of the cellulose component of the cottonfibers.

The cotton warp yarn material herein can be contacted with thecellulose-reactive emulsion copolymer by any suitable technique in orderto form the copolymer-pretreated warp yarn. Such contact can generallyinvolve treatment of the cotton warp yarn material with a treatment bathwhich can be made by diluting an aqueous emulsion copolymer dispersionto a solids content of from about 2.0 wt % to about 10 wt %, morepreferably from about 3 wt % to about 6 wt %. Such treatment baths willalso have a pH of from about 3 to about 7, more preferably from about 5to about 7. Treatment temperature can range from about 45° C. to about60° C.

Warp yarn can be treated with saturating liquors (called “pad baths”)with a nip roll squeeze after each bath saturation. Cotton warp yarn canalso be treated in “package” form with the saturating liquor. Warp yarncan, in fact, be contacted with the pre-treating emulsion copolymertreatment bath using any techniques or equipment which are known in theart for applying liquid material to yarn to incorporate such additivesas sizing, bleach or dye. Thus, for example, the cotton warp yarn can beemulsion copolymer-treated in a sizing machine, a rope dyeing machine, aslasher dyeing machine or a package dyeing machine.

Regardless of the method of application selected, application andprocessing conditions should be selected such that the cotton warp yarnmaterial has a substantially uniform distribution of the emulsioncopolymer associated with it. The warp yarn/copolymer combination willgenerally have a copolymer add-on of from about 1 wt % to about 10 wt %,more preferably from about 3 wt % to about 6 wt %, on a dry basis.

After the warp yarn/copolymer combination has been formed, thiscombination is subjected to curing conditions which are effective tochemically anchor the emulsion copolymer to the cotton fibrous materialwithin the yarn via reaction of the copolymer with at least a portion ofthe hydroxyl moieties of the cellulose component of the cotton fibers.Such chemical reaction can occur via a cross-linking mechanism with thecross-linkable co-monomers which will generally form part of theemulsion copolymer as hereinbefore described. Curing of thefiber/copolymer combination also will generally promote someself-cross-linking of the copolymer within the fibrous cotton materialor the warp yarn as well.

Curing conditions for the warp yarn/copolymer combination will generallyinvolve subjecting the combination to elevated temperatures of fromabout 120° C. to about 150° C. for a period (dwell time) of from about0.2 to about 4 minutes. More preferably, the warp yarn/copolymercombination can be cured by using temperatures of from about 130° C. toabout 145° C. for a period (dwell time) of from about 0.3 to about 1minute. In addition to anchoring the copolymer to the cellulose hydroxylgroups of the cotton fibers within the warp yarn, curing of the warpyarn/copolymer combination will also generally serve to remove waterfrom this combination. Thus curing of the warp yarn/copolymercombination can serve to partially or even substantially completely drythe yarn/copolymer combination prior to the fabric formation and/ordyeing steps of the methods herein.

The treating of the cotton warp yarn material with the emulsioncopolymer and the subsequent curing of the yarn/copolymer combinationserves to provide chemically modified, copolymer-treated cotton yarnmaterial. Such copolymer-treated cotton fibers can then be formed intodenim fabric and dyed using the indigo dyestuff material and dyeingconditions hereinafter described.

Denim Fabric Formation

In accordance with the methods herein, the emulsion copolymer pretreatedcotton warp yarn as hereinbefore described, either before or after beingdyed, is incorporated into cotton denim fabric. The weft or fill yarnalso incorporated into the cotton denim fabric along with the emulsioncopolymer-treated cotton warp yarn will comprise untreated cotton weftyarn. For purposes herein, the cotton weft yarn is “untreated” if it hasnot been modified by the emulsion copolymer treatment procedureshereinbefore described for the warp cotton yarn.

In the denim fabrics herein, not all of the warp yarn needs to beemulsion copolymer-treated cotton yarn and not all of the weft yarnneeds to be untreated cotton yarn. Generally, at least 50% of the warpyarn in the fabric should be emulsion copolymer-treated cotton yarn andat least 50% of the weft yarn should be untreated cotton yarn.Preferably, however, substantially all of the warp yarn in the fabricshould be emulsion copolymer-treated cotton yarn and substantially allof the weft yarn should be untreated cotton yarn.

Cotton warp and weft yarns can be fashioned into cotton denim fabrics inaccordance with the methods herein by any conventional technique knownfor the preparation of such denim fabrics. The method herein iscompatible with cotton denim fabrics having a wide range of fabric basisweights. Cotton denim fabrics will typically have a basis weight rangingfrom about 3 to about 10 oz/yd².

Weaving is a common method for making cotton yarn into cotton denimfabrics. The woven cotton denim fabrics which can be indigo dyed inaccordance with the dyeing methods described hereinafter include, forexample, those of a basic weave, satin weave, twill weave, ripstop weaveor basket weave. Denim fabrics are most commonly of the twill weavetype.

Cotton yarns can also be knitted to provide a variety of denim knitfabric types prior to being dyed in accordance with the dyeing methodherein. Denim knit cotton fabrics will generally be of the warp type,including tricot knits or raschel knits.

Indigo Dyeing Procedures

The emulsion copolymer-treated warp yarns as described herein, or cottondenim fabrics which comprise such emulsion copolymer-treated warp yarn,are dyed in accordance with the methods herein by contacting such yarnsand/or fabrics with an indigo dyestuff material. Indigo has been used todye fabric with “indigo blue” since before recorded history. Indigo hasbeen used in India to dye fabric for at least 4,000 years by methodswhich are practically identical to the methods employed today. Indigowas introduced in Europe in large quantities by the Dutch East IndiaCompany in the early 17th century.

Indigo (C₁₆H₁₀N₂O₂) is the true coloring matter of indigo dye which isgenerically known as Vat Blue 1. When pure, indigo forms a dark, richblue powder or bronzy blue-colored needle crystals. The most importantreaction of indigo is its reaction with reducing agents. When subjectedto a reducing agent in the presence of alkali, indigo combines with twoatoms of hydrogen and is reduced to a colorless body, known asindigo-white or the leuco form, which is insoluble in water, butdissolves in alkali, with a yellow color. The lueco form of the indigodye deposited onto cotton yarn is generally subjected to a “skying”process wherein the lueco form of the dye is oxidized with air to a blueindigo color.

The emulsion copolymer-treated warp yarn of the denim fabrics herein hasa selective affinity for the indigo dyestuff material such that thisemulsion copolymer-treated warp yarn can be indigo-dyed using an aqueousdye liquor comprising a dispersion of the indigo dyestuff material.Highly alkaline conditions are not needed to solubilize the indigo suchthat the aqueous dye liquor can therefore have a pH ranging from about 9to less than about 12, more preferably from about 10 to about 11.

It is also not necessary to convert the indigo to the leuco form suchthat little or no reducing agents need be added to the aqueous dyeliquor. The aqueous dye liquor (i.e., dye bath) can generally have areduction/oxidation potential of from about −400 to about −600 mV.

The aqueous dye liquor used to color the cotton warp yarn material willcomprise sufficient indigo dyestuff material such that contact of thedye liquor with the warp yarn or griege fabric to be dyed will providean amount of dye of from about 15% to about 20% owg (on weight ofgoods). Lower concentrations of the indigo dye in the dye liquor areuseful for tinting operations. Higher dye concentrations in the dyeliquor, of course, produce dyed cotton denim fabrics and garments havingmore intense indigo color. Several passes of the yarn or fabric intocontact with the dye bath may be used to achieve the desired intensityof color.

The aqueous dyeing liquor can optionally contain various fabric treatingadjuvants besides the indigo dyestuff material. Such adjuvants caninclude, for example, optical brighteners, fabric softeners, antistaticagents, antibacterial agents, anti-wrinkling agents, ironing aids,flame-retardants, enzymes, uv stabilizers, anti-foaming agents,perfumes, and the like.

The aqueous dye liquor will generally be contacted with the cotton warpyarn material to be dyed at temperatures of from about 65° C. to about100° C., more preferably from about 80° C. to about 95° C. Under suchdyeing liquor temperature conditions, it is possible to carry out thedyeing procedures of the methods herein at atmospheric pressure.

In accordance with the dyeing method herein, the indigo dyestuffmaterial as hereinbefore described is contacted with thecopolymer-treated cotton warp yarn also hereinbefore described. Suchcontact can occur after the emulsion copolymer-treated cotton warp yarnhas already been incorporated into the cotton denim fabric.Alternatively, the emulsion copolymer-treated cotton warp yarn can becontacted with the indigo dye liquor before this copolymer-treatedcotton yarn is incorporated as warp yarn into the cotton denim fabricsherein.

No matter when the emulsion copolymer-treated warp yarn is contactedwith the indigo dye liquor, such contacting should occur underconditions which are sufficient to affix at least a portion of thecontacted indigo dyestuff material to the outer cross-sectional portionsof the emulsion copolymer-treated cotton warp yarn. Such outercross-sectional portions include primarily the outer, i.e., external,surface of the copolymer-treated cotton warp yarn. But the outer crosssectional portion of the yarn can also include the interior regions ofthe yarn in proximity to the outer surface thereof. The indigo dyestuffmaterial, however, will generally not penetrate the copolymer-treatedwarp yarn material completely, i.e., all the way to the core of theyarn.

The methods herein thus provide an indigo-dyed yarn effect which iscomparable to that of “ring-dyeing” as is described, for example, inU.S. Pat. No. 5,514,187. However, this “ring-dyeing” effect can beachieved with the methods herein with greater precision, morereproducibility, less waste and without the need to utilize hard tocontrol dyeing and skying conditions of conventional ring-dyeingprocedures.

In a preferred embodiment herein, the emulsion copolymer-treated cottonwarp yarn is indigo-dyed after it has been woven or knitted into cottondenim fabric along with untreated cotton weft yarn to form griege denimfabric. Such griege denim fabric can then be dyed using a pad bath incontinuous stenter (open width) frames or with batch processes such as,piece dyeing, jet, beck, jigger or paddle machines. Knit griege goodscan be processed in the same machinery (both continuous and batch) aswoven, just under different conditions.

The griege denim fabric can, of course, be fashioned into end useproducts such as garments, apparel, upholstery, linens, etc. prior tobeing contacted with the aqueous dispersion of the indigo dyestuffmaterial and dyed. For garments, industrial garment washing machines maybe used for dyeing. Optional dyeing application methods include manualprocesses such as spraying or manual wet add-on techniques. For dyeingof griege denim fabrics, garments or other end use articles, dye liquorto fabric ratios of from about 20:1 to about 8:1 can be employed.

Whatever dyeing techniques or apparatus are used for the indigo dying ofthe griege denim fabrics herein, the affinity of the copolymer-treatedwarp yarn in such griege fabrics will result in the indigo dyepreferentially coloring the outer cross-sectional portions of thecopolymer-treated warp yarn to a greater extent than the indigo dyestuffmaterial colors the outer cross-sectional portions of the untreatedcotton weft yarn. This results in the realization of indigo-dyed denimfabrics having a non-uniformly colored appearance.

As an optional step after the indigo dyeing of the griege denim fabricsas hereinbefore described, the resulting indigo-dyed denim fabric can bescoured to thereby preferentially remove indigo dye material which mayhave been to some extent deposited on the untreated cotton weft yarnwithin the dyed fabric. Also the indigo-dyed denim fabric may alsooptionally be further conventionally dyed by contacting the fabric withan additional dyestuff material of the indigo, vat, cationic, azoic,napthol, reactive, direct, sulfur, mordant, disperse or acid type.

In another embodiment, the emulsion copolymer-treated cotton warp yarncan be indigo dyed before it is incorporated into cotton denim fabric.Such indigo dyeing of the copolymer-treated cotton warp yarn can becarried out in the same type of sizing or dyeing apparatus which may beor may have been used for initial treatment of the warp yarn with theemulsion copolymer. Thus, prior to weaving or knitting of thecopolymer-treated warp yarn into denim fabric, the treated warp yarn maybe dyed in a sizing machine, a rope dyeing machine, a slasher dyeingmachine or a package dyeing machine.

Post-Dyeing Operations

The indigo-dyed cotton denim fabrics produced by the dyeing methodsherein can be subjected to any conventional post-dyeing treatment. Onesuch typical post-dyeing operation comprises a further wash down stepwhich serves to remove a portion of dye from all or portions of the dyedfabric. Such a wash down operation gives even new fabrics or garments afashionable worn or used appearance as, for example, with stone-washeddenim blue jeans. Typical wash down techniques involve contacting theindigo-dyed fabrics with an abrasive material such as stones, perlite,pumice, sand and/or diatomaceous earth.

1. A method for preparing indigo-dyed denim fabric, which method comprises: A) preparing emulsion copolymer-treated warp yarn by contacting cotton yarn with an emulsion copolymer and thereafter curing said copolymer to adhere said copolymer to the warp yarn; B) weaving or knitting said emulsion copolymer-treated cotton warp yarn into denim fabric along with untreated cotton yarn as the weft yarn to thereby prepare griege denim fabric having emulsion copolymer-treated cotton warp yarn in the warp direction only; and thereafter C) contacting said griege denim fabric with an aqueous dispersion of an indigo dyestuff material under conditions sufficient to preferentially color the outer cross-sectional portions of said emulsion copolymer-treated warp yarn to a greater extent than said indigo dyestuff material colors the outer cross-sectional portions of said untreated cotton weft yarn, to thereby provide indigo-dyed denim fabric having a non-uniformly colored appearance.
 2. The method according to claim 1 which comprises an additional Step D) of scouring said non-uniformly colored indigo-dyed denim fabric to preferentially remove indigo dye material from the untreated cotton weft yarn within said fabric.
 3. A method for preparing indigo-dyed denim fabric, which method comprises: A) preparing emulsion copolymer-treated warp yarn by contacting cotton yarn with an emulsion copolymer and thereafter curing said copolymer to adhere said copolymer to the warp yarn; B) contacting said emulsion copolymer-treated warp yarn with an aqueous dispersion of an indigo dyestuff material under conditions sufficient to color the outer cross-sectional portions of said emulsion copolymer-treated warp yarn; and C) weaving or knitting said emulsion copolymer-treated, colored warp yarn into denim fabric along with untreated cotton yarn as the weft yarn to thereby prepare denim fabric having emulsion copolymer-treated, colored warp yarn in the warp direction only, to thereby provide indigo-dyed denim fabric having a non-uniformly colored appearance.
 4. The method of claim 1 wherein said non-uniformly colored indigo-dyed denim fabric is subjected to further dyeing by contacting said fabric with an additional dyestuff material of the indigo, vat, cationic, azoic, napthol, reactive, direct, sulfur, mordant, disperse or acid type.
 5. The method of claim 1 wherein said cotton warp yarn is emulsion copolymer-treated in a sizing machine, a rope dyeing machine, a slasher dyeing machine or a package dyeing machine.
 6. The method of claim 1 wherein the emulsion copolymer which is used to treat said cotton warp yarn is, prior to dilution in a yarn treatment bath, in the form of an emulsion having a solids content of from 40 wt % to 65 wt %, and a pH of from 3 to
 7. 7. The method of claim 1 wherein the emulsion copolymer which is used to treat said cotton warp yarn is, prior to contact with said yarn, diluted with water to form a yarn treatment bath having a solids content of from 2.0 wt % to 10 wt %.
 8. The method of claim 7 wherein said cotton warp yarn is contacted with said treatment bath at a temperature of 45° C. to 60° C. and is then cured at a temperature of 120° C. to 150° C.
 9. The method of claim 1 wherein the emulsion copolymer is selected from vinyl ester-based, acrylic-based, styrene/acrylic-based or styrene/butadiene-based emulsion copolymers.
 10. The method of claim 1 wherein the emulsion copolymer comprises a vinyl ester-based copolymer selected from vinyl acetate-ethylene copolymers, vinyl acetate-vinyl versatate copolymers; vinyl acetate-acrylic copolymers, and combinations of said copolymer types, preferably a vinyl acetate-ethylene copolymer comprising from 60 wt % to 95 wt % of vinyl acetate and from 5 wt % to 40 wt % of ethylene, based on total monomers therein.
 11. The method of of claim 1 wherein the emulsion copolymer comprises an acrylic emulsion copolymer which comprises at least two different types of (meth)acrylate co-monomers preferably ethyl acrylate and butyl acrylate co-monomers.
 12. The method of claim 1 wherein the emulsion copolymer comprises from 0.1 wt % to 10 wt %, based on total monomers in the copolymer, of one or more ethylenically unsaturated cross-linking co-monomers having at least one amide, epoxy, or alkoxysilane group.
 13. The method of claim 1 wherein the emulsion copolymer comprises from 0.1 wt % to 10 wt %, based on total monomers in the copolymer, of one of more multifunctional external cross-linking co-monomers selected from diallyl adipate, triallyl cyanurate, butanediol diacrylate, allyl methacrylate, and combinations of said cross-linking co-monomers.
 14. The method of claim 1 wherein the contacting is conducted with an aqueous dye liquor having a pH of 9 to less than 12 and comprising an amount of indigo dyestuff material sufficient to provide 15% to 20% owg of dye on fabric.
 15. The method of claim 1 wherein said indigo-dyed denim fabric is subjected to a further washing step which removes a portion of dye from said dyed denim fabric.
 16. Indigo-dyed cotton denim fabric prepared according to the method of claim
 1. 17. Indigo-dyed cotton denim fabric comprising a plurality of cotton warp yarns and a plurality of cotton weft yarns, said cotton warp yarns being treated with a cellulose reactive emulsion copolymer and then contacted with an aqueous dispersion of an indigo dyestuff material under conditions sufficient to color the outer cross-sectional portions of said emulsion copolymer-treated cotton warp yarns; and said cotton weft yarns being untreated prior to optional contact of said cotton weft yarns with an aqueous dispersion of indigo dyestuff material.
 18. (canceled)
 19. The method of claim 3 wherein the emulsion copolymer is selected from vinyl ester-based, acrylic-based, styrene/acrylic-based or styrene/butadiene-based emulsion copolymers.
 20. The method of claim 3 wherein the emulsion copolymer comprises a vinyl ester-based copolymer selected from vinyl acetate-ethylene copolymers, vinyl acetate-vinyl versatate copolymers; vinyl acetate-acrylic copolymers, and combinations of said copolymer types, preferably a vinyl acetate-ethylene copolymer comprising from 60 wt % to 95 wt % of vinyl acetate and from 5 wt % to 40 wt % of ethylene, based on total monomers therein.
 21. The method of claim 3 wherein the emulsion copolymer comprises an acrylic emulsion copolymer which comprises at least two different types of (meth)acrylate co-monomers preferably ethyl acrylate and butyl acrylate co-monomers.
 22. The method of claim 3 wherein the emulsion copolymer comprises from 0.1 wt % to 10 wt %, based on total monomers in the copolymer, of one or more ethylenically unsaturated cross-linking co-monomers having at least one amide, epoxy, or alkoxysilane group.
 23. The method of claim 3 wherein the emulsion copolymer comprises from 0.1 wt % to 10 wt %, based on total monomers in the copolymer, of one of more multifunctional external cross-linking co-monomers selected from diallyl adipate, triallyl cyanurate, butanediol diacrylate, allyl methacrylate, and combinations of said cross-linking co-monomers.
 24. The method of claim 3 wherein the contacting is conducted with an aqueous dye liquor having a pH of 9 to less than 12 and comprising an amount of indigo dyestuff material sufficient to provide 15% to 20% owg of dye on fabric.
 25. The method of claim 3 wherein said indigo-dyed denim fabric is subjected to a further washing step which removes a portion of dye from said dyed denim fabric.
 26. Indigo-dyed cotton denim fabric prepared according to the method of claim
 3. 